Three-Dimensional Geo-Electrical Structure in Juncalito Geothermal Prospect, Northern Chile

Geothermics 64 (2016) 527–537

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Geothermics

jo urnal homepage: www.elsevier.com/locate/geothermics

Three-dimensional geo-electrical structure in Juncalito geothermal prospect, northern Chile

a,b,∗ a,b

Karin García , Daniel Díaz

Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada 2002, Santiago, Chile

Centro de Excelencia en Geotermia de los Andes (CEGA), Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile

a r a

t i b s

c t

l e i n f o r a c t

Article history: Magnetotelluric (MT) data were recorded to investigate a geothermal prospect in northern Chile. The

Received 30 March 2016

exploration area is located in the Atacama region, at the southern edge of the Chilean Altiplano, at an

Received in revised form 28 July 2016

altitude of ∼4100 masl. This area belongs to the southern end of the Central Volcanic Zone (CVZ), a

Accepted 1 August 2016

segment of the Andean Volcanic Belt associated with the oblique subduction of the Nazca plate beneath

Available online 11 August 2016

the South American plate. In the surveyed area of 19 MT sites, a two-dimensional geometry and an almost

N-S geo-electric strike angle were obtained. A 3-D inversion was carried out and the obtained resistivity

Keywords:

model shows a low resistivity layer (<10 m) thicker westward of the proﬁle. This result agrees with the

Magnetotelluric method

classic pattern of the geo-electric structure in a geothermal system with potential (Anderson et al., 2000;

Central volcanic zone (CVZ)

Spichak and Manzella, 2009). The results from these magnetotelluric measurements were compared with

Geothermal reservoir

Andes different geophysical and geological information gathered in this area, helping to understand the geo-

electrical structure obtained by magnetotellurics and to characterize this geothermal prospect. Different

possible scenarios, given the available data, are shown.

1. Introduction 2000; Spichak and Manzella, 2009; Flócvenz et al., 2005; Ussher

et al., 2000). Alteration is often unaffected by subsequent cool-

The electrical resistivity, obtained by electrical or electromag- ing, and in most cases, the resistivity structure can be regarded

netic methods, is particularly useful in geothermal exploration as a “maximum thermometer” (Árnason et al., 2010). In a classic

because this parameter has strong variation in these systems. The model of a geothermal system, the upﬂow is a zone in the reser-

resistivity is sensitive to temperature, porosity (only if the porosity voir where the ﬂuid ﬂow is predominantly vertical, and generally

has interconnected pores and they are filled with fluid), salinity, the temperature increases with depth. In upflow areas, the base of

and clay content. Conceptually a geothermal system consists of a the conductive clay cap is often elevated because of the relative

heat source, a transport zone for geothermal ﬂuids, an impermeable increase, with temperature, caused by higher resistivity minerals

cap, and sometimes, a reservoir that stores the heat. The imperme- in the mixed layer (Munoz,˜ 2014).

able cap is produced by prolonged ﬂuid circulation reactions on the The Central Volcanic Zone (CVZ) at the Chilean Andes (Fig. 1)

◦ ◦

rock. At temperatures between 70 C and 200 C alteration miner- includes 44 active volcanic ediﬁces, as well as more than 18 active

als such as smectite and illite form. At higher temperatures, over minor centers and/or ﬁelds, and at least 6 potentially active centers

◦

220 C, chlorite-epidote alteration minerals form (Munoz,˜ 2014). and/or caldera systems (Stern, 2004), located in a highly elevated

Some clay minerals, such as smectite and illite-smectite are elec- (>4000 masl) region. In the southern segment of the CVZ (north-

◦

trically conductive as they have loosely bound cations. On the other ern Chile) the Nazca plate is being subducted in a direction of 27

hand, chlorite-epidote has all its ions bound in a crystal lattice and, (Stern, 2004). The modern volcanic front is located in the west-

therefore, they are less electrically conductive than illite-smectite ern Cordillera Occidental, 120 km above the subducted slab and

minerals (Deer et al., 1962; Pellerin et al., 1996; Anderson et al., 240–300 km east of the trench. Crustal thickness reaches ∼70 km

(Asch et al., 2006) and the basement age ranges from late Precam-

brian to Paleozoic. The active volcanoes overlie volcanic rocks of

∗ Late Oligocene to Quaternary age. Many of these older volcanic

Corresponding author at: Centro de Excelencia en Geotermia de los Andes

centers are extremely well preserved because of the hyper-arid

(CEGA), Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza

conditions in the region (Stern, 2004). Andesites, dacytes and rhy-

Ercilla 803, Santiago, Chile.

E-mail address: [email protected] (K. García). olites are the dominant rock type.

http://dx.doi.org/10.1016/j.geothermics.2016.08.001

528 K. García, D. Díaz / Geothermics 64 (2016) 527–537

Fig. 1. Regional map of the Central Volcanic Zone (CVZ). Modiﬁed from Stern (2004).

◦ ◦

In northern Chile (17 S–28 S) during the Pliocene and Quater- expressions are weaker than in other high temperature geothermal

nary periods, an extensional tectonic phase produced differential areas e.g. Hengill geothermal ﬁeld in Iceland (Gasperikova et al.,

block upliftings along N-S trending fault systems. Volcanic vents 2015), Coso geothermal ﬁeld in USA (Newman et al., 2005) or Taupo

and hydrothermal manifestations occur associated with small volcanic zone in New Zealand (Heise et al., 2008; Bertrand et al.,

grabens connected with these fault systems (eg. El Tatio and 2015). However there are blind geothermal ﬁelds that show no

Puchuldiza geothermal ﬁelds), on the western side of the Pliocene- signs of geothermal activity at the surface (Vice et al., 2007; Kratt

Holocene Central Volcanic Zone (Lahsen et al., 2005). In northern et al., 2008).

Chile there are about 90 hot spring areas, and 45 exploration con- In this work we present the results of 3-D resistivity model-

cessions are being surveyed. The most advanced programs have ing obtained with magnetotelluric data (MT) and a comparison

been carried out in the Colpitas, Apacheta, Pampa Lirima, and El with different geophysical and geological information gathered in

Tatio-La Torta geothermal areas. Meanwhile, exploitation conces- this area, which is used to understand the geo-electrical struc-

sions have been granted for the Apacheta and El Tatio La Torta ture obtained from magnetotellurics and to assess the geothermal

Projects, where production wells have been drilled (Lahsen et al., potential of Juncalito prospect.

2015).

The main target of this research, Juncalito geothermal prospect, 2. Geological setting

is located in the Atacama region, 180 km northeast of Copiapo (see

Fig. 2 for details). This area is on the southern side of the CVZ. Even The MT data was collected at Llano los Cuyanos, a ﬂat area at

further south, the absence of astenospheric mantle above the ﬂat approx. 4100 masl., surrounded by alluvial deposits of Pliocene

slab segment precludes the occurrence of arc magmatism (Tassara age to the NW, and alluvial-colluvial deposits of Pleistocene to

et al., 2006). Holocene age to the SE. In the northeast end of the area, the Azufr-

Several indicators such as the closeness to volcanoes and surface era los Cuyanos volcano is located. It belongs to a recent volcanic

expressions of a geothermal ﬁeld (soil with hydrothermal alteration and stratovolcanic complex (Pleistocene to Holocene). On the slope

◦

and high temperature (40.2 C) hot springs) hint at a high geother- of this volcano, hydrothermal alteration is observed. On the south-

mal potential in the Juncalito area (Clavero et al., 1997, 1998). These eastern side of this area there is an older volcanic and stratovolcanic

K. García, D. Díaz / Geothermics 64 (2016) 527–537 529

Fig. 2. Geological setting with MT sites. Modiﬁed from Clavero et al. (1997, 1998).

complex of Pliocene age. On the southwestern slope of this volcano, Similarly, the geomagnetic transfer function (or tipper) is also lin-

hydrothermal alteration is observed. On the western edge of the early related to the vertical and horizontal components of the

study area, the Claudio Gay mountain range is located. Hot water magnetic ﬁeld, indicating lateral resistivity contrasts when plot-

◦

springs at 40.2 C (Risacher et al., 2011) can be found at the southern ted as induction arrows. Furthermore the phase tensor, which is a

edge of the study zone (Río Negro springs, see Fig 2). Cation geother- function of the impedance tensor, was used. Phase tensor param-

mometers measured in Río Negro springs suggest temperatures eter is very useful because, in contrast to the impedance tensor,

◦ ◦

in the range between 101 C and 223 C but this geothermome- it is independent of distortion caused by near surface hetero-

try is of low conﬁdence because of the dilution-mixing process geneities (see more details in Caldwell et al. (2004)). The phase

of alteration waters with brines from the salars. However, a pos- tensor can be represented graphically as ellipses, providing infor-

itive compositional indicator of the Río Negro springs is the silica mation about the dimensionality of the geo-electrical structure.

concentration (130–160 mg/kg SiO2) indicating quartz equilibra- Resistivity gradients are shown by the magnitude of the phase

◦

tion temperatures of 140–160 C (Lira et al., 2012 and references response, the invariant 2 (geometric mean ofthe principal val-

therein). ues max and min of the phase tensor – 2 ( max min)). The

According to the geology of Llano los Cuyanos area (Clavero et al., period is related closely to the depth, with short periods corre-

1997, 1998), we can distinguish between basement and cover, as sponding roughly to shallow depths while long periods correspond

described below. The cover consists of volcanic and unconsolidated to greater depths. For example, if the phase tensor invariant param-

◦

deposits. The volcanic deposits consist of Laguna Verde ignimbrite, eter 2 is larger than 45 at short periods, it implies that the

and below this, it is possible to ﬁnd units of Permian to Miocene electrical conductivity at shallow depths beneath the station is

age, consisting of sedimentary and volcanoclastic deposits. relatively high (with respect to the same station at different peri-

In the study area, other geological and geophysical studies were ods).

performed, helping to describe the Juncalito geothermal prospect. The time series for electric and magnetic ﬁelds measured in

In particular, we discuss soil CO2 surveys, surface temperature, and the study area were processed using robust processing techniques

gravimetry. All these studies will be discussed more thoroughly in based on Egbert and Booker (1986) in order to obtain the impedance

the Complementary Methods section. tensor and the geomagnetic transfer functions.

Data was collected using the ADU07 equipment from Metronix.

3. Magnetotelluric data 19 MT sites (Fig. 2) were obtained in May 2012. Spacing between

measurement sites is between 1 and 2 km.

The magnetotelluric method (MT) linearly relates the horizontal At each site, the horizontal components of both the electric and

electric and magnetic fields to the impedance tensor (also known magnetic fields and the vertical component of the magnetic field

as the transfer function) in the frequency domain. The impedance were measured in order to obtain the transfer function and the

tensor contains information about the resistivity structure. geomagnetic transfer function.

530 K. García, D. Díaz / Geothermics 64 (2016) 527–537

3.1. Dimensionality of the geo-electric structure 3.1.2. Geo-electrical strike

The method described in Smith (1995) was used to obtain

3.1.1. Phase tensor the angle of the geo-electric strike, which considers telluric dis-

Dimensionality of the geo-electric structure was studied before tortion occurring in structures near the surface that are not

making the appropriate inversion (1-D, 2-D, 3-D). Fig. 3 shows one-dimensional. Using distortion matrices, the distorted electric

phase tensor ellipses, at different periods, to determine the dimen- ﬁeld is written as a function of the values the electric ﬁeld would

sionality and orientation of the geo-electric structure, as well as the have if the inhomogeneities were not present.

resistivity gradients. Fig. 4 shows a clear trend of a geo-electric strike, indicating that

◦ ◦

Top left plot at Fig. 3 shows the phase tensor at 31 ms (shallow the best geo-electrical strike is N10 E or N100 E. The result shows

◦

depth). The phase tensor tends to be a circle, indicating that the an ambiguity of ±90 that can be solved using induction arrows,

shallower part of the structure is 1-D. Additionally, these circles which are shown in the following section. A 2-D inversion along a

◦

are red (2 > 45 ), indicating that the structure is conductive at a proﬁle perpendicular to the geo-electrical strike would be suitable

shallow depth. At 695 ms, (top right plot, deeper than 31 ms plot) for most of the dataset, and was carried out along proﬁle A-A (see

the darker red color is only seen in the west, indicating that the Fig. 2 for location of proﬁle). However, as the resistivity structure at

conductive structure reaches greater depth in the western part of depths of interest is likely to be three-dimensional (top left plot in

the survey area. A transition to a more resistive body begins east- Fig. 3), 3-D inversions were performed, showing a good agreement

wards. At 8 s (bottom left plot), the ellipses are oriented almost NS, with the 2-D inverted model.

indicating that the structure is 2-D at this period. Their blue tones

◦

( 2 < 45 ) indicate increasing resistivity below the conductive sur- 3.1.3. Induction arrows

face layer. At 256 s, ellipses remain aligned and 2 increases again Fig. 3 shows four plots of induction arrows at different periods.

with a slight tendency to increase in conductivity at a greater depth. Top left plot is at 31 ms and shows arrows of very small magnitude,

Fig. 3. Phase tensor and induction arrows (real part of geomagnetic transfer function according to Wiese convention, i.e., arrows point outward from a conductor) plotted

at different periods between 0.01 and 1000 s.

K. García, D. Díaz / Geothermics 64 (2016) 527–537 531

3.2. 3-D inversion

We carry out a 3-D instead of a 2-D inversion, as the resistivity

structure at depths of interest is likely to be three-dimensional (top

right plot in Fig. 3) as we will see in this section.

Using data from 19 stations measured in the study area, a 3-D

model was obtained using the ModEM code described by Egbert

and Kelbert (2012) and Kelbert et al. (2014). The inversion proce-

dure is based on Non-Linear Conjugate Gradients (NLCG) and the

program is parallelized (using MPI) in order to work on various pro-

cessors at the same time. For the 3-D case, all the elements of the

impedance tensor are signiﬁcant, and therefore, this method con-

siders the inversion of its four components, plus two elements of

the geomagnetic transfer function, setting an error ﬂoor of 10% for

Fig. 4. Rose diagram showing the strike angle for all sites and periods between 1

◦ ◦

and 10 s. The best geo-electrical strike is N10 E or N100 E. the components inverted. Data at 6 periods per decade were used in

a period range of 0.002 s and 512 s. The study area was discretized

with a grid of 200 m × 200 m in horizontal directions at the cen-

indicating that the horizontal resistivity contrast is minor. This sug- tral part of the model, where all the measured stations are placed,

gest that, at this period, the structure is almost one-dimensional. and increased while moving away from the central part. Various

At a period of 8 s, the arrows are longer and point away from a inversions were performed in order to obtain the smoothest model

main axis oriented NNE-SSW (i.e. there is no more ambiguity in which could ﬁt the data properly. Three slices on the ﬁnal model

the estimated geo-electric strike calculated in the previous section resulting from 3-D inversion are shown in Fig. 5, obtaining a nor-

◦

and the strike value is N10 E), consistently with the occurrence of a malized RMS error of 1.8. For examples of the data ﬁt, see Fig. A.8

geothermal domain. For larger periods, the induction arrows point in the supporting information.

towards the northwest, i.e., away from the volcano. This could be Fig. 5 shows three resistivity proﬁles (see Fig. 2 for location

related to a large conductive structure at depths of several kilome- of proﬁles) obtained from the 3-D modeling. These proﬁles are

ters, just below the volcanic ediﬁces in this area. approximately orthogonal to the geo-electrical strike and have an

Fig. 5. Three slices of the 3-D model obtained from the inversion process. All slices have a depth extension of 4 km, and an approximate horizontal extension of 10 km.

Blue circles indicate location of measured stations. Segmented lines indicate 10 and 100 m values. Structures marked as C1, C2 and R are discussed in the main text. (For

interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

532 K. García, D. Díaz / Geothermics 64 (2016) 527–537

Fig. 6. Diffuse soil CO2 ﬂux in units of [g/m -day]. Modiﬁed from Navarrete (2012).

◦ ◦

orientation of N100 E. The main geo-electric structures are a resis- with anomalous temperatures greater than 15 C (background tem-

◦

tive cover, a conductive layer (C1 and C2) which becomes thicker perature <9 C, during the study period), and aligned NS. (2) To the

at the west (C1) of the proﬁle, and an oval resistive body (R) below south: Río Negro springs (see Fig. 2) with an anomalous tempera-

◦

the conductive layer at the east of the proﬁle. It is possible to iden- ture >30 C, and aligned NE-SW. Additionally, a third anomaly – at

◦

tify conductive structures (C1 and C2). These features are thicker at about 11 C and without surface water expression like the others

the western half of the proﬁle, which is consistent with the results – is located on the west slope of Azufrera de los Cuyanos volcano.

of the phase tensor (see Fig. 3 in 695 ms plot), as the phase tensor This anomaly could be related to a fault-controlled circulation of

ellipses are also indicating a deeper conductive zone to the west meteoric waters or to a thermal aquifer that discharge into the Río

of this area. If one follows the classical resistivity structure of a Negro springs, with a deeper heat source associated with Azufrera

geothermal system, according to Anderson et al. (2000) and Spichak de los Cuyanos and Juncalito volcanoes.

and Manzella (2009), C2 could represent a thinner clay cap with Finally, considering the results of the resistivity structure of the

an elevated base in this part of the study area indicating possible geothermal prospect, it can be suggested that the upﬂow (ascend-

higher temperature alteration at shallower depth. ing ﬂow from the heat source) of the geothermal system could be

in the SW ﬂank of the Azufrera de los Cuyanos volcanic complex.

4. Complementary methods The outﬂow (cooled ﬂow discharge) could take place in Río Negro

springs and in headwaters of Ojos de los Cuyanos stream (Lira et al.,

Other studies in geology and geophysics were performed in 2012).

the study area, which helped with the description of the Juncalito

geothermal prospect and which are discussed below.

4.3. Gravimetry

4.1. Diffuse soil CO2

In Lira (2013) gravimetry measurements and results are

described. Lira (2013) performed a gravimetric study and obtained

Navarrete (2012) studied the diffuse soil CO2 ﬂux (see Fig. 6),

a residual gravity anomaly map (background density: 2.6 g/cc. See

because CO2 is usually one of the most abundant gases in geother-

Fig. 7). On the west side of the MT sites the residual anomaly is

mal systems associated with volcanism. In Juncalito, the CO2 ﬂow

more negative than on the east side. This means that the western

values measured were very low (see Fig. 6). This means that there

layer is of lower density and/or thicker than the eastern one.

isn’t much gas in the ascending geothermal ﬂuid or that there is

The layers associated with low density can be related to rock

some mechanism that prevents its ascent, which may be asso-

with high porosity. In this context and compared with the MT

ciated with an impermeable cover (a seal cap), such as clay-rich

method, an area with layers with high porosity, and where ﬂuids

volcanic deposits (Laguna Verde ignimbrite). Over this cover there

circulation occurs, would give as a result a low resistivity anomaly.

can be found recent volcanic deposits of Juncalito and Azufrera de

This implies that, in this case, low density and/or thicker layers

los Cuyanos volcanoes which, taken together, may be sufﬁciently

are to be found to the west side of the measurements, and also

impermeable to restrict gas upﬂow.

according to the MT method, a low resistivity anomaly can be

observed, being thicker to the west than to the east of the sites.

4.2. Surface temperature

Thus, the results obtained with the gravimetric method support

the MT results.

Lira et al. (2012) performed a study of surface temperature in

Llano los Cuyanos. The study obtained thermal anomalies asso-

ciated with springs and also with areas without hydrothermal 5. Discussion

discharge. Lira et al. (2012) recognized two areas of thermal anoma-

lies with hydrothermal discharge: (1) Headwaters of Ojos de los Different scenarios, that could be described using the available

Cuyanos stream: found to the west of the measurements (see Fig. 2), data, are discussed.

K. García, D. Díaz / Geothermics 64 (2016) 527–537 533

Fig. 7. Residual gravity anomaly map. Background (2600 kg/m ). The pink circles represent the MT sites, black circles are the gravimetric sites and lines are location of MT

proﬁles shown in Fig. 5. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

A possible scenario is that Juncalito prospect is not a geothermal (b) The resistive body R, situated under the updomed base in the

resource and the studied methods indicate that: C2 conductor, could be a reservoir.

With all these data collected in the Juncalito area, this prospect

1. There is not diffuse CO2 ascending.

could be deﬁned as an inferred resource (the thermal energy may

2. The surface temperature anomaly could be related to a fault-

be estimated with a low level of conﬁdence).

controlled circulation of meteoric water.

3. Considering the gravity measurements, R could also be related to

a denser body of a crystalline kind, which would also correspond 6. Conclusions

to the relatively high resistivity values.

4. The conductive anomaly (C1 and C2) would be originated by The geothermal exploration in northern Chile, in particular

ancient evaporitic sequences. And/or the conductive layer may in Juncalito prospect, is difﬁcult because of the possible pres-

still be a hot outﬂow from a distant source that could be outside ence of evaporitic sequences that misrepresent the signal obtained

the measured area from different geological and geophysics methods. For instance,

the results obtained with the cation geothermometers are not

reliable (Gigenbach, 1988) and it is necessary to use other method-

On the other hand, there is evidence for Juncalito being a

ologies. On the other hand, the electrical resistivity signal of a

geothermal resource if we consider the results from:

geothermal system may blend together with the signal of electrical

response from the evaporitic sequences. Therefore it is required

to gather together as much geological and geophysical data as

1. The silica geothermometer, suggesting high temperatures of

◦

>140 C. possible to understand the studied system. However, to get a

better level of conﬁdence concerning the resource, it is essen-

2. Some mechanism that prevents the ascent of diffuse soil CO2,

tial to drill and make direct reservoir measurements from well

which may be associated with an impermeable cover.

data.

3. The surface temperature anomaly that is caused by a deep source

associated with the Azufrera de los Cuyanos and Juncalito volca-

noes.

4. The resistivity structure obtained from the MT method, accord- Acknowledgements

ing which:

(a) The conductivity layer (C1 and C2) may be the response from We would like to thank EASA for their logistic support and per-

clay minerals originated by hydrothermal alteration. mission to carry out this work, in particular to Aldo Giavelli, and the

534 K. García, D. Díaz / Geothermics 64 (2016) 527–537

geological and geophysical service companies: Geovectra, Zonge Appendix A. Data ﬁts

and TRV that partially founded this work. Moreover, this research

was supported by the Andean Geothermal Center of Excellence The following ﬁgures represent the data ﬁt as a comparison

(CEGA), FONDAP project 15090013. We would also like to thank between measured (circles for apparent resistivity and phase, for

Arturo Belmonte, Anne Schlachli, and Fernando Zamudio for their both off-diagonal elements of the impedance tensor), and modeled

ﬁeldwork assistance, as well as Gonzalo Yanez˜ and Diego Morata data (ﬁlled circles for apparent resistivity and phase), for the 19

for their suggestions and proofreading. We would like to thank measured sites. The modeled data was obtained using ModEM soft-

A. Kelbert, G. Egbert and N. Meqbel, for provide acces to ModEM ware trying to ﬁt the full impedance tensor and tipper data for each

inversion routines, and N. Meqbel for provide a model and data site. Error bars of the measured data are smaller than symbol size

visualizer, 3D-Grid. in most cases.

Fig. A.8. Data ﬁt as a comparison between measured (circles for apparent resistivity and phase, for both off-diagonal elements of the impedance tensor), and modeled data

(ﬁlled circles for apparent resistivity and phase), for the 19 measured sites.

K. García, D. Díaz / Geothermics 64 (2016) 527–537 535

Fig. A.8. (Continued)

536 K. García, D. Díaz / Geothermics 64 (2016) 527–537

Fig. A.8. (Continued)

K. García, D. Díaz / Geothermics 64 (2016) 527–537 537

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